Device and method for bonding of substrates

ABSTRACT

A method for bonding a first substrate with a second substrate at respective contact faces of the substrates with the following steps: holding the first substrate to a first sample holder surface of a first sample holder with a holding force F H1  and holding the second substrate to a second sample holder surface of a second sample holder with a holding force F H2 ; contacting the contact faces at a bond initiation point and heating at least the second sample holder surface to a heating temperature T H ; bonding of the first substrate with the second substrate along a bonding wave running from the bond initiation point to the side edges of the substrates, wherein the heating temperature T H  is reduced at the second sample holder surface during the bonding.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/821,139, filed Mar. 17, 2020, which is a continuation of U.S.application Ser. No. 16/667,102, filed Oct. 29, 2019 (now U.S. Pat. No.10,636,662), which is a continuation of U.S. application Ser. No.16/123,494, filed Sep. 6, 2018 (now U.S. Pat. No. 10,504,730, issuedDec. 10, 2019), which is a continuation of U.S. application Ser. No.15/514,182, filed Mar. 24, 2017 (now U.S. Pat. No. 10,109,487, issuedOct. 23, 2019), which is the U.S. National Stage of InternationalApplication No. PCT/EP2016/53270, filed Feb. 16, 2016, said patentapplications herein fully incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a method for bonding a first substrate with asecond substrate.

BACKGROUND OF THE INVENTION

Substrates have been aligned with one another and joined with oneanother for many years in the semiconductor industry. The joining, theso-called bonding, serves to build up a multi-substrate stack. In such amulti-substrate stack, functional units, in particular memory chips,microprocessors, MEMS etc. are connected to one another and thuscombined with one another. Diverse possible applications arise fromthese combination options.

The density of the functional units is increasing year by year. As aresult of advancing technological development, the size of thefunctional units is becoming smaller and smaller. The increasing densityis therefore accompanied by a greater number of functional units persubstrate. This increasing number of units is primarily responsible forthe reduction in parts costs.

The primary drawback with increasingly small functional units is in theincreasing difficulty of producing an error-free, in particular completesuperposition of all the functional units along the bond interface ofthe two substrates.

The greatest problem in present-day alignment technology, therefore, isnot always aligning two substrates with one another, in particular twowafers, with the aid of alignment marks, but rather in producing anerror-free, in particular complete correlation of points of the firstsubstrate with points of a second substrate, said correlation thusextending over the entire area of the substrate. Experience shows thatthe structures on the surfaces of the substrates after the bondingprocess are generally not congruent with one another. A general, inparticular overall, alignment and a following bonding step of twosubstrates are thus not always sufficient to obtain a complete anderror-free congruence of the desired points at every point of thesubstrate surfaces.

There are two fundamental problems in the prior art that stand in theway of a straightforward overall alignment and a subsequent bondingstep.

In the first place, the positions of the structures of the first and/orsecond substrates are generally subject to a deviation from thetheoretical positions. There may be a number of reasons for thisdeviation.

It would be conceivable, for example, that the actual producedstructures deviate from their ideal positions because the productionprocesses were defective or at least involved a tolerance. An example ofthis would be the repeated use of lithography by means of astep-and-repeat process, which during each translational displacement ofthe stamp involves a small, but significant error in the position.

A further, less trivial reason would be the deformation of the substratedue to mechanical, but in particular thermal loading. A substrate has adefined temperature for example at the time of the production of thestructures. This temperature is generally not maintained for the entireprocess sequence of the substrate, but on the contrary changes. Thetemperature change is accompanied by a thermal expansion and thereforein the most ideal case a change in diameter, in the most unfavorablecase a complex thermal deformation.

In the second place, even two substrates which have error-free, inparticular full-area congruence, i.e. overlapping of all structures,shortly before the contacting and the actual bonding process can losethis congruence during the bonding process. The bonding process itselfthus has a decisive influence in the production of an error-freesubstrate stack, i.e. having perfect congruence of the structures.

In the third place, layers and structures that are applied on thesubstrates may generate stresses in a substrate. The layers may forexample be insulation layers, the structures may be through-silicon-vias(TSVs).

One of the greatest technical problems with the permanent bonding of twosubstrates is the alignment accuracy of the functional units between theindividual substrates. Although the substrates can be very preciselyaligned with respect to one another by means of alignment equipment,distortions of the substrates can arise during the bonding processitself. As a result of the distortions thus arising, the functionalunits will not necessarily be correctly aligned with one another at allpositions. The alignment inaccuracy at a specific point on the substratemay be a result of a distortion, a scaling error, a lens error(magnification or reduction error) etc. In the semiconductor industry,all subject areas dealing with such problems are combined under the term“overlay”. A suitable introduction to this subject can be found forexample in: Mack, Chris. Fundamental Principles of OpticalLithography—The Science of Microfabrication. WILEY, 2007, Reprint 2012.

Each functional unit is designed in the computer before the actualproduction process. For example, strip conductors, microchips, MEMS, orany other structure producible with the aid of microsystem technology,are designed in a CAD (computer aided design) program. During theproduction of the functional units, it can however be seen that there isalways a deviation between the ideal functional units designed on thecomputer and the real functional units produced in the clean room. Thedifferences are primarily due to limitations of the hardware, i.e.engineering-related problems, but very often physical limitations. Thus,the resolution accuracy of a structure that is produced by aphotolithographic process is limited by the size of the apertures of thephotomask and the wavelength of the light used. Mask distortions aredirectly transferred to the photoresist. Linear motors of machines canonly approach reproducible positions within a given tolerance, etc. Itis not therefore surprising that the functional units of a substratecannot be exactly identical to the structures designed on the computer.Even before the bonding process, all substrates thus have anon-negligible divergence from the ideal state.

If the positions and/or shapes of two opposite-lying functional units oftwo substrates are compared on the assumption that neither of the twosubstrates is distorted by a bonding process, it is found that ingeneral there is already an imperfect congruence of the functionalunits, since the latter diverge from the ideal computer model due to theerrors described above. The most frequent errors are represented in FIG.8 (Copied from:http://commons.wikimedia.org/wiki/File:Overlay_typical-_model_terms_DE.svg,24.05.2013 and Mack, Chris. Fundamental Principles of OpticalLithography—The Science of Microfabrication. Chichester: WILEY, p. 321,2007, Reprint 2012).

According to the illustrations, a rough distinction can be made betweenoverall and local as well as symmetrical and asymmetrical overlayerrors. An overall overlay error is homogeneous, therefore independentof location. It produces the same divergence between two opposite-lyingfunctional units irrespective of the position. The conventional overalloverlay errors are errors I and II, which arise due to a translation orrotation of the two substrates with respect to one another. Thetranslation or rotation of the two substrates produces a correspondingtranslational or rotational error for all the functional units lyingrespectively opposite one another on the substrates. A local overlayerror arises in a location-dependent manner, mainly due to elasticityand/or plasticity problems and/or preliminary processes, in the presentcase primarily caused by the continuously propagating bonding wave. Ofthe represented overlay errors, errors III and IV are in particularreferred to as “run-out” errors. This error arises primarily due to adistortion of at least one substrate during a bonding process. As aresult of the distortion of at least one substrate, the functional unitsof the first substrate are also distorted in respect of the functionalunits of the second substrate. Errors I and II can however also arisedue to a bonding process, but they are usually superimposed by errorsIII and IV to such a marked extent that it is difficult to detect ormeasure them. This applies to bonders, in particular fusion bonders ofthe latest design, which have an extremely accurate capability for x-and/or y- and/or rotation correction.

There is already a plant in the prior art, with the aid of which localdistortions can be reduced at least partially. It concerns here a localdistortion due to the use of active control elements (WO2012/083978A1).

Initial approaches to a solution for correcting “run-out” errors existin the prior art. US20120077329A1 describes a method for obtaining adesired alignment accuracy between the functional units of twosubstrates during and after the bonding, whereby the lower substrate isnot fixed. The lower substrate is thus not subjected to any boundaryconditions and can bond freely to the upper substrate during the bondingprocess. An important feature in the prior art is, in particular, theflat fixing of a substrate, usually by means of a vacuum device.

In most cases, the arising “run-out” errors become more intensified in aradially symmetrical manner around the contact point, for which reasonthey increase from the contact point to the periphery. In most cases, itinvolves a linearly increasing intensification of the “run-out” errors.Under special conditions, the “run-out” errors can also increasenon-linearly.

Under particularly optimum conditions, the “run-out” errors can beascertained not only by suitable measuring devices (EP2463892), but canalso be described by mathematical functions. Since the “run-out” errorsrepresent translations and/or rotations and/or scaling betweenwell-defined points, they are preferably described by vector functions.Generally, this vector function is a function f:R²→R², i.e. a mappingrule, which maps the two-dimensional definition range of the positioncoordinates onto the two-dimensional value range of “run-out” vectors.Although an exact mathematical analysis of the corresponding vectorfields has not yet been able to be carried out, assumptions are madeconcerning the function properties. The vector functions are, with ahigh degree of probability, at least C^(n) n>=1 functions, i.e. at leastcontinuously differentiable. Since the “run-out” errors increase fromthe contact point to the edge, the divergence of the vector functionwill probably be different from zero. With a high degree of probability,therefore, the vector field is a source field.

The “run-out” error is best ascertained in relation to structures. Astructure is understood to mean any element of a first or secondsubstrate which is to be correlated with a structure on the second orrespectively first substrate. In the case of a structure, it thusrelates for example to alignment marks

corners and/or edges, in particular corners and edges of functionalunits

contact pads, in particular Through Silicon Vias (TSVs) or ThroughPolymer Vias (TPVs)

strip conductors

recesses, in particular holes or depressions

The “run-out” error is generally position-dependent and, in themathematical sense, a displacement vector, between a real and an idealpoint. Since the “run-out” error is generally position-dependent, it isideally specified by vector fields. In the subsequent text, the“run-out” error will, unless stated otherwise, be regarded solely aspunctiform in order to facilitate the description.

“Run-out” error R comprises two partial components.

First partial component R1 describes the intrinsic part of the “run-out”error, i.e. the part that can be traced back to the defective productionof the structures or a distortion of the substrate. It is thereforeinherent in the substrate. It should be noted that a substrate can alsohave an intrinsic “run-out” error when the structures have beencorrectly produced at a first temperature, but the substrate is subjectto a temperature change to a second temperature prior to the bondingprocess and thermal expansions thus occur, which distort the entiresubstrate and therefore also the structures located thereon.

Temperature differences of a few Kelvin, sometimes even a tenth of aKelvin, are already sufficient to generate such distortions.

Second partial component R2 describes the extrinsic part of the“run-out” error, i.e. the part that is not caused until the bondingprocess. It is not present before the bonding process. This includesprimarily local and/or overall distortions of the first and/or secondsubstrate due to forces acting between the substrates, which can lead toa deformation in the nanometer range.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for bondingtwo substrates, with which the bonding accuracy is increased as far aspossible at every position of the substrates. It is a further object ofthe invention to provide a method with which an error-free, inparticular full-area congruence of the structures of two substrates canbe produced.

Drawbacks of the prior art are solved with the features of theindependent claim(s). Advantageous developments of the invention aregiven in the sub-claims. All combinations of at least two features givenin the description, in the claims and/or the drawings also fall withinthe scope of the invention. In the stated value ranges, values lyinginside the stated limits are also deemed to be disclosed as limitingvalues and can be claimed in any combination. If an individual or aplurality of method steps can be performed on different devices ormodules, the latter are in each case disclosed separately as anindependent method.

The idea underlying the invention is to reduce a heating temperatureT_(H) already during the bonding and to switch off heating during thebonding. Heating temperature T_(H) serves in particular to produce atemperature sufficient for the bonding at a bonding face of thesubstrates. An important aspect of a further embodiment according to theinvention includes removing the fixing of a substrate, in particularduring the bonding, in order to permit free deformability of thesubstrate stack to the bonded. A further important aspect of a thirdembodiment according to the invention includes the possibility of thesubstrate stack, in particular its interface, being ventilated orsubjected to pressure during the bonding.

As a first and/or second substrate wafers are particularly qualified.

A characteristic process according to the invention during the bonding,in particular permanent bonding, preferably fusion bonding, is the mostconcentric, punctiform possible contacting of the two substrates. Inparticular, the contacting of the two substrates can also take place ina non-concentric manner. The bonding wave being propagated from anon-concentric contact point would reach different sides of thesubstrate edge at different times. The complete mathematical-physicaldescription of the bonding wave behavior and of the resultant “run-out”error compensation would be correspondingly complicated. In particular,the contacting point will not be located far from the centre of thesubstrate, so that the effects possibly resulting therefrom arenegligible, at least at the edge. The distance between a possiblenon-concentric contacting point and the centre of the substrate ispreferably less than 100 mm, with preference less than 10 mm, withgreater preference less than 1 mm, with greatest preference less than0.1 mm, with the greatest possible preference less than the 0.01 mm. Inthe further description, contacting should as a rule be understood tomean concentric contacting. In the broader sense, centre is preferablyunderstood to mean the geometrical centre-point of a basic ideal body,if necessary compensated for asymmetries. In the case of wafers standardin the industry with a notch, the centre is therefore the circlecentre-point of the circle that surrounds the ideal wafer without anotch. In the case of wafers standard in the industry with a flat(flattened side), the centre is the circle centre-point of the circlethat surrounds the ideal wafer without a flat. Analogous considerationsapply to arbitrarily shaped substrates. In specific embodiments, it mayhowever be useful for the centre to be understood as the centre ofgravity of the substrate. In order to ensure an exact, concentricpunctiform contacting, an upper holding device (sample holder) providedwith a central bore and a pin movable in a translational manner thereinis provided with a radially symmetrical fixing. The use of a nozzle,which uses a fluid, preferably a gas, instead of the pin for theapplication of pressure, would also be conceivable. Furthermore, the useof such elements can even be completely dispensed with, if devices areprovided which can cause the two substrates to approach one another by atranslational movement, with the further provision that at least one ofthe two substrates, preferably the upper substrate, has an impressedcurvature in the direction of the other substrate on account ofgravitation, and therefore automatically makes contact during theaforementioned translational approach, with a sufficiently small spacingwith respect to the corresponding second substrate.

The radially symmetrical fixing/holding involves either provided vacuumholes, a circular vacuum lip or comparable vacuum elements, with the aidof which the upper substrate can be fixed. The use of an electrostaticholding device is also conceivable. The pin in the central bore of theupper sample holder serves for the controllable deflection of the fixed,upper substrate.

In a further embodiment according to the invention, the holding deviceis designed in such a way that the first and/or second substrate iscurved in a convex and/or concave manner by a generated over- and/orunder-pressure in the sample holder. For this purpose, vacuum tracksand/or cavities through which fluids can flow or which can be evacuatedare preferably provided in the holding device. The use of a nozzle forthe pinpoint application of pressure can be dispensed with in favor of apressure building up globally. According to the invention, embodimentsare conceivable wherein the substrate is sealed and/or otherwise fixed,in particular at the edge. If, for example, a holding device is designedwhich generates an under-pressure relative to the external atmosphere, aseal at the edge of the substrate is sufficient. If an over-pressure isgenerated inside the holding device in order to curve the substrateoutwards, i.e. in a convex manner, the substrate is preferably fixed atthe edge, in particular mechanically. As a result of the application ofan under-pressure or over-pressure on the substrate from the underside,the curvature of the substrate can be adjusted exactly.

After the contacting of the centers of the two substrates has takenplace, the fixing of the upper sample holder is released, in particularin a controlled and gradual manner. The upper substrate drops down, onthe one hand due to gravity and on the other hand due to a bonding forceacting along the bonding wave and between the substrates. The uppersubstrate is bonded to the lower substrate radially from the centre tothe side edge. A formation of a radially symmetrical bonding waveaccording to the invention thus arises, which runs in particular fromthe centre to the side edge. During the bonding process, the twosubstrates press the gas, in particular air, present between thesubstrates ahead of the bonding wave and thus ensure a bonding interfacewithout gas inclusions. The upper substrate virtually lies on a kind ofgas cushion during the descent.

The first/upper substrate is not subject to any additional fixing afterthe initiation of the bond at a bond initiation point, i.e. it can movefreely apart from the fixing at the bond initiation point and can alsobecome distorted. As a result of the bonding wave advancing according tothe invention, the stress states arising at the bonding wave front andthe prevailing geometrical boundary conditions, each circle segment,infinitesimally small compared to its radial thickness, is subject to adistortion. However, since the substrates represent rigid bodies, thedistortions add up as a function of the distance from the centre. Thisleads to “run-out” errors, which are intended to be eliminated by themethod according to the invention and the device according to theinvention.

The invention thus relates to a method and a device for reducing or evencompletely preventing the “run-out” error between two bonded substratesduring bonding, in particular by thermodynamic and/or mechanicalcompensation mechanisms. Furthermore, the invention deals with acorresponding article, which is produced with the device according tothe invention and the method according to the invention.

The “run-out” error is in particular dependant on the position on thesubstrate along the substrate surface. It emerges in particular that the“run-out” error increases from the centre to the periphery of thesubstrate. Such radially symmetrical run-outs occur mainly in the caseof fusion-bonded substrates, which are contacted centrally by means of apin and the bonding wave whereof propagates automatically, in particularradially, after the contacting.

The “run-out” error is particularly dependent on the speed of thebonding wave. The “run-out” error will generally be greater, the higherthe bonding wave speed. According to the invention, therefore, bondingwave speeds are preferably set which are less than 100 mm/s, preferablyless than 50 mm/s, still more preferably less than 10 mm/s, mostpreferably less than 1 mm/s, most preferably of all less than 0.1 mm/s.In a particular embodiment according to the invention, the bonding wavespeed is detected by measuring means.

The “run-out” error is particularly dependent on the spacing (gap)between the two substrates immediately before the start of the(pre-)bonding process. If the upper, first substrate, in particular, isdeformed by deformation means with a first force F₁, the spacing betweenthe substrates is a function of the location. In particular, the spacingbetween the substrates is greatest at the edge. The minimum spacing islocated in the region of the convex maximum of the deformed substrate.The shape of a deformed substrate thus also has an influence on the“run-out” error. The spacing between the substrates at the edge(substrate edge spacing D) directly before the bonding is in particularless than 5 mm, preferably less than 2 mm, still more preferably lessthan 1 mm, most preferably less than 0.5 mm, most preferably of all lessthan 0.1 mm. The spacing between the substrates beneath the convexmaximum directly before the bonding is in particular less than 1 mm,preferably less than 100 μm, still more preferably less than 10 μm, mostpreferably less than 1 μm, most preferably of all less than 100 nm.

The “run-out” error is particularly dependent on the type and shape ofthe sample holder and the resultant fixing/holding of the respectivesubstrate. Publication WO2014/191033A1 discloses a number of embodimentsof preferred sample holders, to which reference is made in this regard.In the processes disclosed, a release of the substrate from a sampleholder after the removal of the fixing, in particular vacuum fixing, isof crucial importance. The surface roughness of the sample holder isselected as great as possible, its waviness as small as possible. Agreat surface roughness ensures as few contact points as possiblebetween the sample holder surface and the substrate. The separation ofthe substrate from the sample holder therefore takes place with minimumexpenditure of energy. The waviness is preferably minimal, in order notto create new sources of a “run-out” due to the sample holder surface.It is pointed out that the comments concerning the waviness do not meanthat the surface of the sample holder may not also be curved as a whole.

The roughness is indicated as an average roughness, quadratic roughnessor as an average roughness depth. The averaged values for the averageroughness, the quadratic roughness and the average depth generallydiffer for the same measurement section or measurement area, but lie inthe same range in terms of order of magnitude. The following numericalvalue ranges for the roughness are therefore to be understood either asvalues for the average roughness, the quadratic roughness or for theaverage roughness depth. The roughness is set in particular greater than10 nm, preferably greater than 100 nm, more preferably greater than 1μm, most preferably greater than 10 μm, most preferably of all greaterthan 100 μm. The “run-out” error is particularly dependent ontime-related aspects. A bonding wave propagating too quickly does notgive the material of the substrates enough time shortly after and/or atand/or before the bonding wave to bond together in the optimum manner.It may therefore also be of crucial importance to control the bondingwave in a time-dependent manner.

The “run-out” error is particularly dependent on the loading procedureof the substrate on the sample holder. When the substrate is loaded andfixed, a distortion of the substrate may occur which is maintained bythe fixing and is also introduced into the substrate stack during the(pre-)bond. The substrate is therefore transferred from an end effectoronto the sample holder as far as possible without distortion.

The “run-out” error is particularly dependent on temperature differencesand/or temperature fluctuations between the two substrates. Thesubstrates are fed in particular from different process steps ordifferent process modules to the bonding module. Different processes mayhave been carried out at different temperatures in these processmodules. Furthermore, the upper and lower sample holder may have adifferent structure, a different type of design and therefore differentphysical, especially thermal, properties. It is conceivable for examplethat the thermal masses and/or the thermal conductivities of the sampleholders differ from one another. This leads to a different loadingtemperature or to a different temperature at the time of the (pre-)bond.The sample holders for performing the processes according to theinvention are therefore preferably provided with heating and/or coolingsystems, so that the temperature of at least one substrate (preferablyboth) can be adjusted exactly. In particular, it is conceivable to adaptthe temperatures of the two substrates to different values, so that thesubstrate is thermally distorted overall by at least one of the twosubstrates being subjected to a thermal action. The adaptation of asubstrate to a desired initial state is thus achieved, in particular tocompensate for “run-out” error component R1.

The “run-out” error is particularly dependent on the ambient pressure.The effects of the ambient pressure have been discussed and disclosedcomprehensively in WO2014/191033A1. Reference is made thereto in thisregard.

The “run-out” error is particularly dependent on a symmetry of thesystem, so that preferably as many components as possible (still morepreferably at least the overwhelming part) are build up and/or arrangedsymmetrically. In particular, the thicknesses of the substrates aredifferent. Furthermore, different layer sequences of different materialswith different mechanical properties may be present on each substrateand may have to be taken into account. Furthermore, one of thesubstrates is preferably deformed, whereas the other substrate lies flaton the sample holder. All the properties, parameters and embodimentswhich lead to an asymmetry being present have in particular an effect onthe “run-out” error. Some of these asymmetries cannot be avoided. Thus,the thicknesses of substrates, the layers on the substrates and thefunctional units are defined by the process and customer specifications.According to the invention, an attempt is made to minimize as far aspossible, in particular to completely eliminate, the “run-out” inparticular by varying other variable parameters.

In particular, the “run-out” error is position-dependent. The aim of themeasures according to the invention is, in particular, to obtain at eachposition a “run-out” error which is smaller than 10 μm, is preferablysmaller than 1 μm, is still more preferably smaller than 100 nm, is mostpreferably smaller than 10 nm, is most preferably of all smaller than 1nm.

The sample holders that are preferably used for the embodimentsaccording to the invention comprise fixing means. The fixing means serveto fix the substrates with a fixing force or with a corresponding fixingpressure. The fixing means can in particular be the following:

mechanical fixing means, in particular clamps, or

vacuum fixing means, in particular with

individually controllable vacuum tracks or

vacuum tracks connected to one another, or

electrical fixing means, in particular electrostatic fixing means, or

magnetic fixing means or

adhesive fixing means, in particular

Gel-Pak fixing means or

fixing means with adhesive, in particular controllable, surfaces.

The fixing means are in particular electronically controllable. Thevacuum fixing means is the preferred kind of fixing. The vacuum fixingmeans preferably comprises a plurality of vacuum tracks, which emerge atthe surface of the sample holder. The vacuum tracks are preferablyindividually controllable. In a technically preferred application,several vacuum tracks are united to form vacuum track segments which areindividually controllable, i.e. can be evacuated or flooded separately.Each vacuum segment is preferably independent of the other vacuumsegments. The possibility of constituting individually controllablevacuum segments is thus obtained. The vacuum segments are preferablydesigned annular. A targeted, radially symmetrical fixing and/or releaseof a substrate from the sample holder, in particular performed from theinside outwards, is thus enabled or vice versa.

Possible sample holders are disclosed in publications WO2014/191033A1,WO2013/023708A1, WO2012/079597A1 and WO2012/083978A1. Reference is madethereto in this regard.

During at least one, preferably all the processes steps according to theinvention, it is advantageous to detect the advance of the bonding waveor at least the state of the bonding wave and thus to ascertain thelatter at specific times. For this purpose, measuring means arepreferably provided, in particular with cameras. The monitoringpreferably takes place by means of:

cameras, in particular visual cameras or infrared cameras,

and/or

conductivity measuring devices.

If the determination of the position of the bonding wave takes placewith the aid of a camera, the position of the bonding wave, inparticular the course of the bonding wave, can be detected at any time.The camera is preferably an infrared camera, which digitalizes the dataand relays the latter to a computer. The computer then enables theevaluation of the digital data, in particular the determination of theposition of the bonding wave, the size of the bonded area or furtherparameters.

A further possibility for monitoring the advance of the bonding waveincludes a measurement of the surface conductivity, which changes withan advancing bonding wave. For this purpose, the prerequisites for sucha measurement must be present. The measurement of the surfaceconductivity takes place in particular by contacting of two electrodesat two opposite-lying positions of a substrate. In a particularembodiment according to the invention, the electrodes contact the edgeof the substrates, wherein they do not hinder the bonding of thesubstrates at the edge. In a second, less preferred embodiment accordingto the invention, the electrodes are withdrawn from the surface beforethe bonding wave reaches the side edge of the substrates.

Processes are described below, wherein these processes preferablyproceed in the described sequence, in particular as separate steps.Unless the description states otherwise, the process steps anddisclosures are each transferable from one embodiment to the other, ifthis is technically capable of performance by the person skilled in theart.

In a first process step of a first embodiment of the method according tothe invention, two substrates are positioned and fixed on sampleholders, one on a first/upper sample holder, the second on asecond/lower sample holder. The feeding of the substrates can take placemanually, but preferably by means of a robot, i.e. automatically. Theupper sample holder preferably comprises deformation means for thetargeted, in particular controllable deformation of the upper, firstsubstrate with a first force F₁. The upper sample holder comprises inparticular at least one opening, through which a deformation means, inparticular a pin, can bring about the mechanical deformation of theupper, first substrate. Such a sample holder is disclosed for example inpublication WO2013/023708A1.

In a second process step, the deformation means, in particular a pin,contacts the rear side of the upper, first substrate and produces aslight deformation, in particular a deflection, indicated as concavefrom the side of the deformation means (i.e. from above). Thedeformation means acts on the first substrate in particular with a firstforce F₁ of more than 1 mN, preferably more than 10 mN, still morepreferably more than 50 mN, most preferably more than 100 mN, but inparticular less than 5000 mN. The force is too small to release theupper first substrate from the sample holder, but strong enough toproduce the deflection according to the invention. The force preferablyacts as far as possible in a punctiform manner on the substrate. Since apunctiform action does not actually exist, the force preferably acts ona very small area. The area is in particular less than 1 cm², preferablyless than 0.1 cm², still more preferably less than 0.01 cm², mostpreferably of all less than 0.001 cm². In the case of action on an areaof 0.001 cm², the acting pressure according to the invention is inparticular greater than 1 MPa, preferably greater than 10 MPa, stillmore preferably more than 50 MPa, and most preferably greater than 100MPa, and most preferably of all greater than 1000 MPa. The disclosedpressure ranges also apply to the other areas disclosed above.

In a third process step, a relative approach of the two substrates, inparticular through the relative approach of the sample holders, inparticular takes place. Preferably, the lower sample holder is raised,so that the lower, second substrate actively approaches the upper, firstsubstrate. The active approach of the upper sample holder towards thelower sample holder or the simultaneous approach of both sample holderstowards one another is however also conceivable. The approach of the twosubstrates takes place in particular up to a spacing between the 1 μmand 2000 μm, preferably between 10 μm and 1000 μm, still more preferablybetween 20 μm and 500 μm, and most preferably between 40 μm and 200 μm.The spacing is defined as the smallest vertical distance between twosurface points of the substrates.

Before the bonding or pre-bonding, or contacting, the first and/or thesecond substrate are heated by heating means and/or cooled by coolingmeans, i.e. temperature-regulated.

In a fourth process step, a further application of force on the upper,first substrate takes place. In a first approach according to theinvention, the first substrate is acted upon by a second force F2 of thedeformation means of in particular more than 100 nM, preferably morethan 500 mN, still more preferably more than 1500 mN, most preferablymore than 2000 mN, most preferably of all more than 3000 mN. The firstcontacting of the upper, first substrate with the lower, secondsubstrate is thus brought about and at least supported. The calculationof the preferably occurring pressures takes place by dividing the forceby a minimally adopted area of 0.001 cm².

In a fifth process step, a deactivation of the heating means inparticular takes place, in particular heating of the lower sampleholder, arranged in particular integrally in the lower sample holder.

In a sixth process step, monitoring of the propagation of the advancingbonding wave in particular takes place (see also above “monitoring ofthe bonding wave”). The monitoring tracks the advance of the bondingwave and therefore the advance of the bonding process especially over aperiod of more than 1 s, preferably more than 2 s, still more preferablymore than 3 s, most preferably more than 4 s, most preferably of allmore than 5 s. Instead of tracking/controlling the bonding process overa time interval, tracking of the bonding wave can also be indicated viathe, in particular radial, position of the bonding wave. The tracking ofthe bonding process takes place in particular until the bonding wave isat a radial position which corresponds to at least 0.1 times, preferablyat least 0.2 times, still more preferably at least 0.3 times, mostpreferably at least 0.4 times, most preferably of all 0.5 times thediameter of the substrate. If tracking of the bonding advance is to bemeasured by means of conductivity measurements over the surface, thebonding advance can also take place via the percentage proportion of thebonded or non-bonded surface. The monitoring of the bonding advanceaccording to the invention then takes place in particular until morethan 1%, preferably more than 4%, still more preferably more than 9%,most preferably more than 16%, most preferably of all more than 25% ofthe area has been bonded. Alternatively, the monitoring takes placecontinuously.

A control of the process sequence preferably takes place on the basis ofdefined/set or settable values from the monitoring, which lie within theaforementioned value ranges. A first waiting time for the advance of thebonding wave and up to the initiation of the next process step resultstherefrom.

In a seventh process step, switching-off of the fixing means of theupper, first sample holder in particular takes place. It would also beconceivable for the upper, first substrate to be released by a targetedremoval of the fixing. Especially in the case of vacuum fixing means,which comprise a plurality of individually controllable vacuum tracks,the targeted removal of the fixing takes place by means of a continuousremoval of the vacuum, in particular from the centre to the edge. Theseventh process step is initiated in particular at a point in time t₁,at which one of the parameters of the measuring means reaches adefined/set or settable value (see in particular sixth process step).

More generally, or to put it another way, holding force F_(H1) isreduced during the bonding at a point in time t₁, in particular to anextent such that the first substrate is released from the first sampleholder.

In an eighth process step, renewed or ongoing monitoring of thepropagation of the advancing bonding wave in particular takes place bythe measuring means. The monitoring tracks the advance of the bondingwave and therefore the advance of the bonding process, preferably over aperiod of more than 5 s, preferably more than 10 s, still morepreferably more than 50 s, most preferably more than 75 s, mostpreferably of all more than 90 s. Instead of tracking the bondingprocess over a time interval, the tracking of the bonding wave can alsobe measured via the, in particular radial, position of the bonding wave.The tracking of the bonding process takes place in particular until thebonding wave is at a radial position which is at least 0.3 times,preferably at least 0.4 times, more preferably at least 0.5 times, mostpreferably 0.6 times, most preferably of all 0.7 times the diameter ofthe substrate. If tracking of the bonding advance is to be possible bymeans of conductivity measurements over the surface, the bonding advancecan also take place via the percentage proportion of the bonded ornon-bonded surface. The monitoring of the bonding advance according tothe invention takes place until more than 9%, preferably more than 16%,still more preferably more than 25%, most preferably more than 36%, mostpreferably of all more than 49% of the area has been bonded.Alternatively, the monitoring takes place continuously.

A control of the process sequence preferably takes place on the basis ofdefined/set or settable values from the monitoring, which lie within theaforementioned value ranges. A second waiting time for the advance ofthe bonding wave and up to the initiation of the next process stepresults therefrom.

In a ninth process step, the application of the deformation means isstopped. If the deformation means is a pin, the pin is withdrawn. If thedeformation means is one or more nozzles, the fluid flow is interrupted.If the deformation means are electrical and/or magnetic fields, thelatter are switched off. The ninth process step is initiated inparticular at a point in time at which one of these parameters of themeasuring means reaches a defined/set or settable value (see inparticular eighth process step).

In a tenth process step, renewed or ongoing monitoring of thepropagation of the advancing bonding wave takes place. The monitoringtracks the advance of the bonding wave and therefore the advance of thebonding process, preferably over a period of more than 5 s, preferablymore than 10 s, still more preferably more than 50 s, most preferablymore than 75 s, most preferably of all more than 90 s. Instead oftracking the bonding process over a time interval, the tracking of thebonding wave can also be indicated via the, in particular radial,position of the bonding wave. The tracking of the bonding process takesplace in particular until the bonding wave is at a radial position whichis at least 0.6 times, preferably at least 0.7 times, more preferably atleast 0.8 times, most preferably 0.9 times the diameter of thesubstrate. If the substrate has an edge profile, it is not possible tofollow the tracking of the bonding process up to the outermost edge,since approx. 3-5 mm is not bonded on account of the edge profile. Iftracking of the bonding advance is to be possible by means ofconductivity measurements over the surface, the bonding advance can thenalso take place via the percentage proportion of the bonded ornon-bonded surface. The monitoring of the bonding advance according tothe invention takes place until more than 36%, preferably more than 49%,still more preferably more than 64%, most preferably more than 81%, mostpreferably of all more than 100% of the area has been bonded.Alternatively, the monitoring takes place continuously.

A control of the process sequence preferably takes place on the basis ofdefined/set or settable values from the monitoring, which lie within theaforementioned value ranges. A third waiting time for the advance of thebonding wave and up to the initiation of the next process step resultstherefrom.

An example of a process sequence of the first embodiment is reproducedbelow:

Loading of the substrates

Bringing of the pin into contact with the wafer (force on the wafer 100mN), without starting the bond

Relative approach of the two wafers towards one another (spacing 40-200μm) Application of force on the wafer in order to initiate a fusion bondbetween the two substrates (force 1500-2800 mN)

Deactivation of heating Waiting until the bonding wave has propagatedsufficiently far (typically 1-5 s)—waiting time 1.

Switching-off (exhaust) of the top wafer holding vacuum (in particularboth zones simultaneously)

Waiting until the bonding wave has propagated further (in particular2-15 s)—waiting time 3

Withdrawal of the pin

Waiting until the bonding wave has propagated completely (in particular5-90 s)—waiting time 4

The individual process steps can be generalized by the general technicalteaching described above.

The process according to the second embodiment corresponds to the firstembodiment from the first up to and including the seventh process step.

In an eighth process step, a reduction of the holding force orswitching-off of the fixing means of the lower, second sample holdertakes place. It would also be conceivable for the lower, secondsubstrate to be released by a targeted removal of the fixing. Especiallyin the case of vacuum fixing means, which comprises a plurality ofindividually controllable vacuum tracks, the targeted removal of thefixing takes place preferably by means of a continuous removal of thevacuum, in particular from the centre to the edge. The eighth processstep according to the invention is an important procedure for reducingthe “run-out” error. By means of the reduction of the holding force orswitching-off of the fixing means of the lower, second sample holder,the lower/second substrate is able, according to the invention, to adaptto the upper first substrate. The removal of the fixing results as itwere in a removal of an additional (mathematical-mechanical) boundarycondition which would restrict the process of bonding.

More generally, or to put it another way, holding force F_(H2) isreduced during the bonding at a point in time t2, in particular to anextent such that the second substrate is able to be deformed on thesecond sample holder.

A ninth process step corresponds to the eighth process step of the firstembodiment.

In a tenth process step according to the invention, the second, inparticular already partially bonded, substrate is fixed again on thelower, second sample holder. The tenth process step according to theinvention is also an important procedure for reducing the “run-out”error. By means of a renewed fixing, in particular a renewedswitching-on of the holding vacuum, the progress of the bonding is againrestricted by the (mathematical-mechanical) boundary condition.

More generally, or to put it another way, holding force F_(H2) isincreased at a point in time t4, in particular after the bonding.

An eleventh corresponds to the ninth and a twelfth corresponds to thetenth process step according to the first embodiment.

In a very special embodiment according to the invention, theswitching-off of the fixing means according to process step 8 and therenewed switching-on of the fixing means according to process step 10can be repeated several times before the completion of the bondingprocess. In particular, it is even possible to carry out theswitching-off and renewed fixing in a location-resolved manner. Thisfunctions according to the invention primarily with the individuallycontrollable vacuum tracks or vacuum segments already mentioned in thedisclosure. In the most ideal case, therefore, a location-resolvedand/or time-resolved removal or fixing of the lower/second substrate iscarried out.

An example of the process sequence of the second embodiment isreproduced below:

Loading of the substrates Bringing of the pin (deformation means) intocontact with the wafer (in particular force on the wafer of 100 mN),without starting the bond—pin force 1

Relative approach of the two wafers towards one another (in particular,spacing 40-200 μm)—spacing 1

Pressing on the wafer in order to initiate a fusion bond between the twosubstrates (in particular force 1500-2800 mN)—pin force 2

Deactivation of heating

Waiting until the bonding wave has propagated sufficiently far (inparticular 1-5 s)—waiting time 1

Switching-off (exhaust) of the holding vacuum for the upper wafer (inparticular both zones simultaneously)

Switching-off (exhaust) of the holding vacuum for the lower wafer

Waiting until the bonding wave has propagated further (in particular2-15 s)—waiting time 3

Switching-on of the holding vacuum for the lower wafer—vacuum 1

Withdrawal of the pin

Waiting until the bonding wave has propagated completely (in particular5-90s)—waiting time 4

The individual process steps can be generalized by the general technicalteaching described above.

The process according to the third embodiment corresponds to the secondembodiment from the first up to and including the ninth process step. Inthe ninth process step, the parameters are preferably set 10 to 40%lower than in the second embodiment. The waiting time until the tenthprocess step is thus reduced, wherein an additional waiting time isintroduced, or the second waiting time is split up, in the thirdembodiment.

In a tenth process step according to the invention, ventilation of thespace between the lower, second sample holder and the lower/secondsubstrate lying thereon virtually unfixed takes place at a definedpressure. Pressure is understood here to mean the absolute pressure. Anabsolute pressure of 1 bar corresponds to atmospheric pressure. In orderto carry out the process according to the invention, therefore, thechamber must be previously evacuated and then opened to the atmosphere,i.e. ventilated. The free mobility of the substrate is thus promoted, sothat distortions with respect to the first substrate are furtherminimized. The pressure amounts in particular to between 1 mbar and 1000mbar, preferably between 2.5 mbar and 800 mbar, still more preferablybetween 5 mbar and 600 mbar, most preferably between 7.5 mbar and 400mbar, most preferably of all between 10 mbar and 200 mbar. In a furtherembodiment according to the invention, it is conceivable that theembodiment according to the invention takes place under atmosphericpressure up to the mentioned tenth process step and thereafter anover-pressure is generated in the chamber by means of a compressor. Thepressure in this case lies in particular between 1 bar and 3 bar,preferably between 1 bar and 2.5 bar, still more preferably between 1bar and 2 bar, most preferably between 1 bar and 1.5 bar, mostpreferably of all between 1 bar and 1.2 bar.

In an eighth process step, renewed or ongoing monitoring of thepropagation of the advancing bonding wave in particular takes place bythe measuring means. The monitoring tracks the advance of the bondingwave and therefore the progress of the bonding process over a period ofmore than 1 s, preferably more than 2 s, still more preferably more than5 s, most preferably more than 10 s, most preferably of all more than 15s. Instead of tracking the bonding process over a time interval, thetracking of the bonding wave can also be indicated via the, inparticular radial, position of the bonding wave. The tracking of thebonding process takes place until the bonding wave is at a radialposition which is at least 0.3 times, preferably at least 0.4 times,more preferably at least 0.5 times, most preferably 0.6 times, mostpreferably of all 0.7 times the diameter of the substrate. If trackingof the bonding progress is to be measurable by means of conductivitymeasurements over the surface, the bonding progress can also take placevia the percentage proportion of the bonded or non-bonded surface. Themonitoring of the bonding progress according to the invention takesplace until more than 9%, preferably more than 16%, still morepreferably more than 25%, most preferably more than 36%, most preferablyof all more than 49% of the area has been bonded. Alternatively, themonitoring takes place continuously.

A control of the process sequence preferably takes place on the basis ofdefined/set or settable values from the monitoring, which lie within theaforementioned value ranges. A first waiting time for the advance of thebonding wave and up to the initiation of the next process step resultstherefrom.

In a twelfth process step according to the invention, the second, inparticular already partially bonded, substrate is again fixed on thelower, second sample holder.

More generally, or to put it another way, holding force F_(H2) isincreased at a point in time t4, in particular to after the bonding.

A thirteenth corresponds to the ninth and a fourteenth corresponds tothe tenth process step of the first embodiment.

An example of a process sequence of the third embodiment is reproducedbelow:

Loading of the substrates

Bringing of the pin into contact with the wafer (in particular, force onthe wafer 100 mN), without starting the bond—pin force 1

Relative approach of the two wafers towards one another (in particular,spacing 40-200 μm)—spacing 1

Pressing on the wafer in order to initiate a fusion bond between the twosubstrates (in particular force 1500-2800 mN)—pin force 2

Deactivation of heating

Waiting until the bonding wave has propagated sufficiently far (inparticular 1-5 s)—waiting time 1

Switching-off (exhaust) of the holding vacuum for the upper wafer (inparticular both zones simultaneously)

Switching-off (exhaust) of the holding vacuum for the lower wafer

Waiting until the bonding wave has propagated further (in particular1-10 s)—waiting time 2

Ventilation of the volume between the lower wafer and the chuck (lowersample holder) for a defined period at a defined pressure (in particular10-200 mbar)—pressure 1

Waiting until the bonding wave has propagated further (in particular2-15 s)—waiting time 3

Switching-on of the holding vacuum for the lower wafer—vacuum 1

Withdrawal of the pin

Waiting until the bonding wave has propagated completely (in particular5-90 s)—waiting time 4.

The individual process steps can be generalized by the general technicalteaching described above.

The described processes can be continued, in particular in furtherprocess modules.

In a first, conceivable continuation, the produced substrate stack canbe investigated, in particular in a metrology module. The investigationchiefly comprises measurements of the bond interface for thedetermination of:

alignment errors, in particular

overall alignment errors and/or

run-out errors and/or

defects, in particular

voids and/or

bubbles and/or

cracks

If the inspection of the substrate stack reveals non-tolerable errors,the substrate stack is preferably separated again. The separationpreferably takes place using processes and equipment which have beendisclosed in publications EP 2697823B1 and WO2013/091714A1. Reference ismade thereto in this regard. The investigation of the bond interfacetakes place in particular before a further heat treatment.

In a second, conceivable continuation, the produced substrate stack isheat-treated. The heat treatment leads in particular to a strengtheningof the produced bond between the substrates of the substrate stack. Theheat treatment takes place in particular at a temperature higher than25° C., preferably higher than 100° C., more preferably higher than 250°C., most preferably higher than 500° C., most preferably of all higherthan 750° C. The temperature essentially corresponds to heatingtemperature T_(H). The produced bond strength is in particular greaterthan 1.0 J/m², preferably greater than 1.5 J/m², more preferably greaterthan 2.0 J/m², and most preferably of all greater than 2.5 J/m². Theheat treatment preferably takes place under vacuum. The vacuum pressureis in particular less than 1 bar, preferably less than 800 mbar, stillmore preferably less than 10⁻³ mbar, most preferably less than 10⁻⁵mbar, most preferably of all less than 10⁻⁸ mbar.

It is also conceivable, however, that the heat treatment is carried outin a protective gas atmosphere. This is particularly advantageous whenthe protective gases used facilitate the heat transfer. The thermalconductivity of the protective gases is in particular greater 0 W/(m*K),preferably greater than 0.01 W/(m*K), more preferably greater than 0.1W/(m*K), most preferably greater than 1 W/(m*K). The thermalconductivity of helium lies for example between approx. 0.15 W/(m*K) and0.16 W/(m*K). The protective gases are in particular:

noble gases, in particular helium, neon, argon, krypton and/or xenon

molecular gases, in particular carbon dioxide and/or nitrogen

any combinations of the aforementioned gases

The substrates preferably have approximately identical diameters D1, D2,which diverge from one another in particular by less than 5 mm,preferably less than 3 mm, still more preferably less than 1 mm.

According to a further, in particular independent, aspect of theinvention, the deformation takes place by mechanical actuating meansand/or by temperature control of the first and/or second holdingdevices.

The deformation according to the invention can be implemented moreeasily by the fact that the first substrate and/or the second substrateis/are fixed exclusively in the region of the side walls at the firstand/or second holding faces.

The results of the processes according to the invention are dependent ona multiplicity of physical parameters which can be directly attributedto the substrates or the surroundings. In the following text of thedisclosure, the most important parameters and their influence on the“run-out” error are described. The parameters are roughly split up intosingle parameters and pair-parameters. A single parameter cannot beassigned to a symmetric side, in particular not to a substrate. Apair-parameter can have, on one symmetric side, in particular on a firstsubstrate, a different value from on the respective opposite symmetricside, in particular on a second substrate. There is a first, uppersymmetric side and a second, lower symmetric side. An example of asingle parameter is bonding wave speed v or gas (mixture) pressure p. Anexample of a pair-parameter is substrate thicknesses d1 and d2.

When the influence of a pair-parameter on the bonding result isdescribed in the following text, it is assumed, unless stated otherwise,that all the other values of each pair-parameter are preferablyidentical. The following example is mentioned by way of example. Whenthe influence of two different substrate thicknesses d1 and d2 on thebonding result is described, it is assumed that both moduli ofelasticity E1 and E2 of the two substrates are identical.

The aim is to minimize or completely eliminate the “run-out” error by anoptimum, calculated and/or empirically determined, in particulartime-dependent, bending line. A bending line is understood to mean thesymmetry-reduced representation of a one-dimensional function, whichmaps the surface positions of a substrate, the substrate surface, as afunction of the location coordinate, in particular of a radialcoordinate. Symmetry-reduced means that, on account of the radiallysymmetrical symmetry of the two substrates, the calculation of theone-dimensional bending line is sufficient to allow conclusions to bedrawn regarding the two-dimensional contacting of the substrates withone another that produces the mentioned minimal or completely eliminated“run-out” error. In simple terms, the bending line of a substrate can bedescribed as the substrate surface in particular facing the bondinterface. The description of the bending line for the first substratepreferably applies similarly to the second substrate.

The bending lines, i.e. the substrate surfaces, are decisivelyinfluenced according to the invention especially by one or more of thefollowing parameters.

Substrate thicknesses d1, d2 are associated by volumes V1 and V2 anddensities p1, p2 with masses m1, m2 and therefore gravitational forcesG1, G2 of the two substrates. Gravitational force G1 of the firstsubstrate has a direct influence on the acceleration behavior of thefirst, upper substrate in the direction of the second, lower substrate.When the lower fixing means is switched off, gravitational force G2 is ameasure of the inertial force of the second, lower substrate andtherefore a measure of the effort of the second, lower substrate to movealong the bonding wave towards the first, upper substrate or to holdthereto or to remain lying thereon.

Moduli of elasticity E1, E2 are a measure of the stiffness of thesubstrates. They contribute decisively to the bending line and thereforealso define the function with the aid of which the way in which thesubstrates move towards one another can be described.

Forces F1 and F2 have an influence on the area with which the twosubstrates are bonded together, in particular concentrically. Sincethere is a punctiform contacting only in the ideal case, it always hasto be assumed that the contacting of the two substrates takes place atan area in the centre. The size of the area is determined decisively byforces F1 and F2. The size of the contact area is decisive for theboundary condition.

The overall thermal expansion state of the substrates can be influencedby temperatures T1, T2 of the two substrates. According to theinvention, it is thus possible to ascertain the extent to which thesubstrates are deformed by thermal expansion relative to a referencetemperature. The correct temperature regulation of the upper and/orlower substrate is therefore an essential aspect for a “run-out”compensation that is as correct and complete as possible. Thetemperatures of the two substrates can preferably be set differently.The temperatures are set in particular in such a way that the substratesare in a state of expansion in which the structures to be bondedtogether are congruent with one another, i.e. the “run-out” errordisappears (on the assumption that an additional “run-out” error doesnot arise during bonding due to the parameters already mentioned above).The temperatures required for this can be determined by measuring meansand/or empirically.

Gas (mixture) pressure p influences the resistance that is presented bythe atmosphere against the substrates moving towards one another. Adirect influence on bonding wave speed v can be exerted by the gas(mixture) pressure. Reference is made in this regard to publicationWO2014191033 A1.

Holding forces F_(H1), F_(H2) serve primarily for the fixing of thesubstrates before the actual bonding process. Holding force F_(H1) is aboundary condition for process steps 1 to 6 inclusive, but loses theinfluence of a boundary condition determining the bending line when thefixing means is switched off. Likewise, holding force F_(H2) is used asa boundary condition only for the point in time of an active, lowerfixing. Thus, new conditions must accordingly be formulated at thelatest from process step 7 for elasticity-theoretical calculations.

Initial curvature radii r₁₀, r₂₀ are the initial radii of the substratesbefore the process according to the invention. They are functions of thelocation, but in particular are constant relative to the location. In aparticular first embodiment according to the invention, initialcurvature radius r₁₀ of the second, lower substrate is infinitely large,since the second lower substrate lies flat at the start of the processaccording to the invention. In a further particular second embodimentaccording to the invention, initial curvature radius r₁₀ of the second,lower substrate is a finite, positive or negative constant,corresponding to a constant convex or concave curvature. In this case,the second lower substrate is present in a shape curved convex orconcave at the start of the process according to the invention. One suchsample holder is described in publication WO2014191033A1, to whichreference is made in this regard. In particular, at least initialcurvature radius r₁₀ of the second, lower substrate corresponds to asample holder curvature radius of the surface of the second, lowersample holder on which the second substrate lies.

Substrate curvature radii r1, r2 of the two substrates along the bondingwave are a result of the solution of the elasticity-theoreticalequations taking account of the stated parameters. They are inparticular functions of location and time.

The bonding wave speed is a result of the aforementioned parameters.

Further advantages, features and details of the invention emerge fromthe following description of preferred examples of embodiment and withthe aid of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a diagrammatic cross-sectional representation (not true toscale) of a first process step of a first embodiment of a methodaccording to the invention,

FIG. 1b shows a diagrammatic cross-sectional representation (not true toscale) of a second process step,

FIG. 1c shows a diagrammatic cross-sectional representation (not true toscale) of a third process step,

FIG. 1d shows a diagrammatic cross-sectional representation (not true toscale) of a fourth process step,

FIG. 1e shows a diagrammatic cross-sectional representation (not true toscale) of a fifth process step,

FIG. 1f shows a diagrammatic cross-sectional representation (not true toscale) of a sixth process step,

FIG. 1g shows a diagrammatic cross-sectional representation (not true toscale) of a seventh process step,

FIG. 1h shows a diagrammatic cross-sectional representation (not true toscale) of an eighth process step,

FIG. 1i shows a diagrammatic cross-sectional representation (not true toscale) of a ninth process step,

FIG. 1j shows a diagrammatic cross-sectional representation (not true toscale) of a tenth process step,

FIG. 2 shows a diagrammatic cross-sectional representation (not true toscale) of an additional process step of a third embodiment of a methodaccording to the invention,

FIG. 3 shows a diagrammatic cross-sectional representation (not true toscale) of an optional additional process step, and

FIG. 4 shows a diagrammatic cross-sectional representation (not true toscale) of two substrates.

DETAILED DESCRIPTION OF THE INVENTION

Identical components and components with the same function are denotedby the same reference numbers in the figures.

FIG. 1a shows a first process step, wherein a first, in particularupper, substrate 2 has been fixed to a sample holder surface 1 o of afirst, in particular upper, sample holder 1. The fixing takes place bymeans of fixing means 3 with a holding force F_(H1).

First sample holder 1 has an, in particular central, through-opening, inparticular bore 4. The through-opening is used to pass through adeformation means 4 for deforming first substrate 2.

In an advantageous embodiment shown here, first sample holder 1comprises holes 5, through which an observation of the bonding progresscan take place by measuring means. Hole 5 is preferably an elongatedmilled-out portion.

A second substrate 2′ is loaded and fixed on a second, in particularlower sample holder 1′. The fixing takes place by means of fixing means3′ with a holding force F_(H2).

Fixing means 3, 3′ are preferably vacuum fixing means.

Sample holders 1, 1′ in particular comprise heating 11 (heating means).For the sake of clearer illustration, heating 11 is representeddiagrammatically only in second, lower sample holder 1′ in the figures.

All the stated parameters or forces, which describe or influence theproperties of substrates 2, 2′, are generally functions of locationand/or time.

Temperatures T1 and T2 of the two substrates 2, 2′ are mentioned as anexample of a parameter. Temperatures T1 and respectively T2 cangenerally be location-dependent, for which reason temperature gradientsexist. In this case, it is expedient to indicate the temperatures asexplicit functions of location and/or time.

The two gravitational forces G1 and G2 are mentioned as an example of aforce. In the figures, they represent the total gravitational forcesacting on substrates 2, 2′. It is however perfectly clear to the personskilled in the art that the two substrates 2, 2′ can be split up intoinfinitesimal (dimensional) parts dm and that the influence ofgravitation can be related to each of these dimensional parts dm. Thegravitational force should therefore generally be indicated as afunction of location and/or time.

Similar considerations apply to all the other parameters and/or forces.

FIG. 1b shows a second process step according to the invention, whereindeformation means 4, in particular a pin, exerts a pressure on a rearside 2 i of first substrate 2 in order to bring about a deformation offirst substrate 2. The deformation of first substrate 2 takes place witha first force F₁.

In a process step according to FIG. 1c , a relative approach of the twosample holders 1, 1′ and therefore of the two substrates 2, 2′ towardsone another takes place up to a defined spacing. The approach can alsotake place during or before the second process step.

In a process step according to FIG. 1d , the initiation of the bond, inparticular a pre-bond, takes place with a second force F₂. Second forceF₂ provides for a further, in particular infinitesimally smalldeflection and a further approach of the two substrates 2, 2′ andfinally contacting at a contact point 7.

A bonding wave, more precisely a bonding wave front 8, starts topropagate in particular in a radially symmetrical manner, preferablyconcentrically, from contact point 7 at a bonding wave speed v. In thecourse of the further process steps, bonding wave speed v can change, sothat bonding wave speed v can be defined as a function of location (orof time). Bonding wave speed v can be influenced by various measures.

In a further process step according to FIG. 1e , heating 11 of firstand/or second sample holder 1, 1′ is switched off and further heating offirst and/or second substrate 2, 2′ is thus interrupted. In a furtherprocess step according to FIG. 1f , monitoring of bonding wave front 8takes place with the aid of measuring means 9, in particular at leastone optical system, preferably an infrared optical system. Through the(at least one—the number preferably corresponds to the number of opticalsystems) hole 5, measuring means 9 can detect substrate rear side 2 i offirst substrate 2, more preferably the bond interface between the twosubstrates 2, 2′, and thus bonding wave front 8. The detection of thebond interface takes place in particular at measuring means 9 which aresensitive to electromagnetic radiation, which can penetrate the twosubstrates 2, 2′ without significant weakening. A light source 12 ispreferably positioned above and/or below and/or inside sample holder 1‘, the electromagnetic radiation whereof illuminates and/or shinesthrough sample holder 1’ and/or substrates 2, 2′ and can be detected bymeasuring means 9. The images thus taken are preferably black-and-whiteimages. The brightness differences permit an unequivocal identificationof the bonded regions from the non-bonded regions. The transition regionbetween the two regions is the bonding wave. By means of such ameasurement, it is possible in particular to determine the position ofbonding wave front 8 and therefore, especially for a plurality of suchpositions, also bonding wave speed v.

FIG. 1g shows a further, seventh process step, wherein fixing means 3 offirst sample holder 1 is released by the fact that holding force F_(H1)is at least reduced. If fixing means 3 is a vacuum fixing means, morepreferably a vacuum fixing means with a plurality of separatelycontrollable vacuum segments (with a plurality of holding forcesF_(H1)), the release takes place in particular from the inside outwardsby means of a targeted switching-off of the vacuum segments (orreduction of holding force/holding forces F_(H1)) from the insideoutwards.

FIG. 1h shows a further process step, wherein bonding wave front 8 ismonitored by measuring means 9 after the release from first substrateholder 1.

FIG. 1i shows a further process step, wherein the action of deformationmeans 6 on first substrate 2 is interrupted. If deformation means 6 is amechanical deformation means, in particular a pin, the interruptiontakes place by a withdrawal. When nozzles are used, the interruptiontakes place by switching off the fluid flow. In the case of electricaland/or magnetic fields, the interruption takes place by switching offthe fields.

FIG. 1j shows a further process step, after which the two substrates 2,2′ are completely bonded together. In particular, further monitoring ofbonding wave front 8 (no longer depicted, since the bond has alreadybeen completed in this process state) takes place in this process stepwith the aid of measuring means 9 up to the end of the bond, at which asubstrate stack 10 formed from first and second substrates 2, 2′ iscompleted.

FIG. 2 shows an optional process step, wherein, in particular after theprocess step according to FIG. 1g , a reduction of holding force F_(H2)of second, lower fixing means 3′ of second, lower sample holder 1′ takesplace. In particular, holding force F_(H2) is reduced to 0, i.e. thefixing is deactivated. The effect of this, in particular, is that secondsubstrate 2′ can move unhindered, in particular in the lateral directionalong lower sample holder surface 1 o′.

In a further, advantageous embodiment, second substrate 2′ is raisedalong bonding wave front 8 to an extent such that it is raised, inparticular locally, from second, lower sample holder 2′. This is broughtabout, in particular, by applying pressure to second substrate 2′ fromsecond sample holder 1′.

Gravitational force G2 counteracts the lifting of second substrate 2′throughout the bonding process and thus also influences the contactingof the two substrates 2, 2′ and therefore the “run-out”.

FIG. 3 shows an optional process step according to the invention,wherein the chamber in which the process according to the inventionproceeds is ventilated before the production of the completely bondedsubstrate stack 10. The ventilation serves in particular to controladvancing bonding wave front 8. A precise description of the possibleways of exerting an influence is disclosed in publicationWO2014/191033A1, to which reference is made in this regard. Theventilation takes place with a gas or gas mixture. In particular, theventilation takes place by opening a valve to the surroundingatmosphere, so that the chamber is ventilated with the surrounding gas(mixture). Subjecting the chamber to an over-pressure with a gas or gasmixture is also conceivable, instead of ventilation to the surroundingatmosphere.

FIG. 4 shows a diagrammatic representation (not to scale) of twosubstrates 2, 2′, which are defined by a plurality of parameters.Substrate surfaces 2 o, 2 o′ correspond to the bending lines of firstupper substrate 2 and respectively second, lower substrate 2′ at adefined point in time. They are defined decisively by the aforementionedparameters. Their shape changes as a function of time during the bondingprocess according to the invention.

LIST OF REFERENCE NUMBERS

-   1 o, 1 o′ sample holder surfaces-   2, 2′ substrates-   2 o, 2 o′ substrate surfaces-   2 i substrate rear side-   3, 3′ fixing means-   4 bore-   5 holes-   6 deformation means-   7 contact point-   8 bonding wave front-   9 measuring means-   10 substrate stack-   11 heating-   12 light source-   F₁, F₂ force-   F_(H1), F_(H2) holding force-   v bonding wave speed-   T_(H) heating temperature-   T1, T2 substrate temperatures-   E1, E2 substrate moduli of elasticity-   d1, d2 substrate thicknesses-   V1, V2 substrate volumes-   m1, m2 substrate masses-   p1, p2 substrate densities-   G1, G2 substrate gravitational forces-   r1, r2 substrate curvature radii-   r10, r20 initial substrate curvature radii-   D substrate edge spacing

Having described the invention, the following is claimed:
 1. A device for bonding a first substrate with a second substrate, comprising: a holding device configured to curve the first substrate, the second substrate, or a combination thereof in a convex manner or a concave manner through an over-pressurizing or an under-pressurizing therein; and a measuring means configured to detect a bonding wave to enable control of the bonding wave.
 2. The device according to claim 1, wherein the measuring means is further configured to detect a back side of the first substrate.
 3. The device according to claim 1, wherein the measuring means is further configured to measure the bonding wave.
 4. The device according to claim 1, further comprising: a first substrate holder configured to hold the first substrate with a first holding force F_(H1); and a second substrate holder configured to hold the second substrate with a second holding force F_(H2), and wherein at least one of the first and second holding forces F_(H1) and F_(H2) is reduced to 0 to control the bonding wave.
 5. The device according to claim 1, wherein the measuring means is further configured to detect a state of the bonding wave and an advance of the bonding wave.
 6. The device according to claim 1, wherein the measuring means is further configured to detect at least one of a position of the bonding wave and a size of a bonded area.
 7. The device according to claim 1, wherein the measuring means comprises at least one of optics and cameras.
 8. The device according to claim 1, wherein the measuring means comprises conductivity measurement means.
 9. The device according to claim 4, wherein the holding device is further configured to impose first and second holding forces F_(H1) and F_(H2) to respectively fix the first and second substrates to the first and second substrate holders, and wherein the holding device is further configured to reduce the first and second holding forces F_(H1) and F_(H2) and respectively control releasing of the first and second substrates from the first and second substrate holders.
 10. The device according to claim 9, wherein the holding device comprises first and second vacuums respectively applied from the first and second substrate holders, the first and second vacuums being configured to impose first and second holding forces F_(H1) and F_(H2) to respectively fix the first and second substrates to the first and second substrate holders, the first and second vacuums being further configured to reduce the first and second holding forces F_(H1) and F_(H2) and respectively control releasing of the first and second substrates from the first and second substrate holders.
 11. The device according to claim 1, wherein the bonding wave is detected over a period that is greater than 1 second.
 12. The device according to claim 1, wherein the measuring means is further configured to detect the bonding wave via a radial position of the bonding wave corresponding to at least 0.1 times respective diameters of the first and second substrates.
 13. The device according to claim 1, wherein the measuring means is further configured to detect the bonding wave via a percentage amount of a bonded surface of the first and second substrates with respect to a non-bonded surface of the first and second substrates.
 14. The device according to claim 1, wherein the bonding wave is controlled in view of a defined set of values obtained by the measuring means.
 15. The device according to claim 1, wherein the bonding wave is controlled by targeted removal of a fixing means by continuous removal of a vacuum.
 16. A method for bonding a first substrate with a second substrate, comprising: curving the first substrate, the second substrate, or a combination thereof in a convex manner or a concave manner through an over-pressurizing or an under-pressurizing therein; detecting a bonding wave between the first substrate and the second substrate during contact therewith; and controlling the detected bonding wave.
 17. The method according to claim 16, wherein the detecting of the bonding wave further comprises detecting a back side of the first substrate.
 18. The method according to claim 16, further comprising: holding the first and second substrates by a holding device, the holding device comprising a first sample holder and a second sample holder, the first sample holder holding the first substrate with a first holding force F_(H1), the second sample holder holding the second substrate with a second holding force F_(H2), wherein at least one of the first and second holding forces F_(H1) and F_(H2) is reduced to 0 to control the bonding wave.
 19. The method according to claim 16, wherein detecting of the bonding wave further comprises detecting at least one of a state of the bonding wave and an advance of the bonding wave.
 20. The method according to claim 16, wherein the detecting of the bonding wave further comprises detecting at least one of a position of the bonding wave and a size of a bonded area. 